The present invention relates to a method of and an apparatus for controlling an engine power train including an engine for a vehicle which can operate on a lean air-fuel ratio and which drives the vehicle through an automatic transmission.
Active efforts have been made to develop an engine capable of operating on a lean air-fuel ratio larger than a stoichiometric air-fuel ratio to improve fuel consumption, while also suppressing harmful matters products included in exhaust gas from the engine. A method of controlling the air-fuel ratio previously proposed in Japanese Application Laid-open No.52825/1978 changes the stoichiometric air-fuel ratio to a lean air-fuel ratio when the operating condition of the engine is within a predetermined range. This method increases the air-fuel ratio gradually after the load on the engine has increased beyond a predetermined value and decreases the air-fuel ratio gradually to an initial air-fuel ratio when the load decreases. Another method previously proposed in Japanese Application Laid-open No.10224/1976 increases the air-fuel ratio when the load on the engine is in a middle-load region, and controls the air-fuel ratio so that a fuel mixture at the stoichiometric air-fuel ratio or a rich air-fuel ratio is supplied to the engine when the load is in a low-load region or a high-load region, attaching importance to the output power of the engine.
Since the fuel amount is reduced below that of the stoichiometric air-fuel ratio during operation at a lean air-fuel ratio, the engine torque decreases when the air-fuel ratio is increased with the opening of the throttle valve which has been kept fixed. Since the catalyst for reducing the nitrogen oxide (NOx) concentration in the exhaust gas does not function effectively in a state where the air-fuel ratio is other than the stoichiometric air-fuel ratio, lean-burn operation is possible only at the air-fuel ratio that makes the NOx concentration of the exhaust gas lower than an allowable limit; that is, the transition of the air-fuel ratio between the stoichiometric air-fuel ratio and the air-fuel ratio for lean-burn operation must be completed as quickly as possible to satisfy the requirements for a clean exhaust gas.
It is desirable to use an automatic transmission in combination with such an engine to secure easy driving. The gear changing operation of an automatic transmission is controlled according to a shift schedule stored in a memory of a controller. The shift schedule is designed on the basis of the opening of the throttle valve, which is an index of the engine torque, and the vehicle speed (xe2x80x9cMotor Fanxe2x80x9d, San-ei Shobo, p. 29, December, 1990). The lockup schedule also is designed on the basis of the opening of the throttle valve and the vehicle speed. The line pressure is controlled according to the opening of the throttle valve so as to transmit the output power of the engine efficiently to the drive shaft.
As mentioned above, the transition between the stoichiometric air-fuel ratio and the air-fuel ratio for lean-burn operation must be quickly completed. However, since the difference between the torque produced when the engine is operating at the stoichiometric air-fuel ratio and the torque produced when the engine is operating at a lean air-fuel ratio for lean-burn operation is about 30%, the vehicle may be shocked and the driver feels that the output power of the engine is insufficient during lean-burn operation, if the air-fuel ratio is changed quickly.
A first object of the present invention is to provide an engine power train control method and control apparatus capable of quickly changing the air-fuel ratio so as to meet the requirements for the exhaust gas content without causing shocks and reducing the engine torque.
Since the engine torque decreases if the opening of the throttle valve is kept fixed while the engine is operating on the lean air-fuel ratio, the timing of changing the speed of the automatic transmission, the timing of lockup and the setting of the line pressure become inappropriate, and shocks due to changing of the speed increase and the operability is deteriorated.
A second object of the present invention is to provide an engine power train control method and control apparatus capable of suppressing shocks due to a changing of the speed during lean-burn operation and of securing satisfactory operability.
With the first object in view, the present invention carries out a transition between the air-fuel ratio for lean-burn operation and the stoichiometric air-fuel ratio by varying an air-fuel ratio command value stepwise, and controls the air amount to change the air amount supplied to the engine stepwise during transition between lean-burn operation and non-lean-burn operation.
The air-fuel ratio command value is determined by retrieving data previously stored in the controller according to the basic fuel amount and the engine speed and interpolating the data. The data to be used depends on whether or not a current air-fuel ratio command value specifies a lean air-fuel ratio. When retrieving and interpolating the data, either the basic fuel amount or the engine speed, or both the basic fuel amount and the engine speed; are rounded off to an optional place before retrieval and interpolation.
In the transition between the air-fuel ratio for lean-burn operation and the stoichiometric air-fuel ratio, the air amount is controlled so that the fuel amount immediately before the transition and the fuel amount after the completion of the transition are equal to each other. When the engine is in a lean-burn operation, the air amount is determined by retrieving data previously stored in the controller according to the position of the accelerator pedal, which is actuated by the driver, and the engine speed, and by interpolating the data. When the engine is in a non-lean-burn operation, the air amount is determined only on the basis of the position of the accelerator pedal.
In a fixed period following the stepwise change of the air-fuel ratio command value, the basic fuel amount is varied stepwise from a value determined on the basis of the air amount and the engine speed to a value determined on the basis of the position of the accelerator pedal and the engine speed. If the position of the accelerator pedal changes to a position outside a predetermined range during the fixed period, the basic fuel amount is determined on the basis of the air amount and the engine speed.
With the second object in view, the present invention uses selectively a gear changing pattern for a lean-burn operation and a gear changing pattern for a non-lean-burn operation.
The gear changing operation and the lockup operation are controlled according to the position of the accelerator pedal, the engine speed and the vehicle speed, and the line pressure of the automatic transmission is determined according to the position of the accelerator pedal and the engine speed while the engine is in a lean-burn operation.
Concerning a system of in which the air amount changes according to changes in the air-fuel ratio, the gear changing operation and the lockup operation are controlled according to the position of the accelerator pedal and the vehicle speed, and the line pressure of the automatic transmission is determined according to the position of the accelerator pedal.
When the temperature of the oil in the automatic transmission is lower than a predetermined temperature, or when it is found that a part of the automatic transmission is out of order, the air-fuel ratio command value is inhibited from specifying a lean air-fuel ratio.
Since the air-fuel ratio command value is varied stepwise during transition between the air-fuel ratio for lean-burn operation and the stoichiometric air-fuel ratio, the air-fuel ratio can be quickly changed. Since the air amount is varied stepwise, the variation of the engine torque due to a change in the air-fuel ratio can be suppressed without delay.
Since whether or not the current air-fuel ratio command value specifies a lean air-fuel ratio is taken into consideration when retrieving data for determining the air-fuel ratio, hunting between the lean air-fuel ratio and the non-lean air-fuel ratio due to change in the basic fuel amount caused by a change in the air amount does not occur. Since either the basic fuel amount or the engine speed, or both the basic fuel amount and the engine speed, are rounded off to an optional place before the retrieval and interpolation of data, the air-fuel ratio can be varied smoothly in the range for lean-burn operation, so that the combustion in the engine can be stabilized.
Since the air amount is controlled so that the fuel amount immediately before the transition between the air-fuel ratio for lean-burn operation and the stoichiometric air-fuel ratio and the fuel amount after the completion of the transition are equal to each other, the engine torque remains constant regardless of the transition of the air-fuel ratio.
Since the air amount for lean-burn operation is determined by retrieving data previously stored in the controller according to the position of the accelerator pedal and the engine speed and interpolating the data, and the air amount for non-lean-burn operation is determined uniquely according to the position of the accelerator pedal, the engine torque can be properly corrected at every operating point of the engine during lean-burn operation, and the performance of the engine at every operating point is the same as that of non-lean-burn engine, when a non-lean air-fuel ratio is specified.
Since the basic fuel amount is changed stepwise from a value determined on the basis of the air amount and the engine speed to a value determined on the basis of the position of the accelerator pedal and the engine speed for a fixed period following the stepwise change of the air-fuel ratio command value, an optional fuel amount can be determined when the air amount is changed stepwise.
Since the basic fuel amount is determined according to the air amount and the engine speed when the position of the accelerator pedal changes to a position outside a predetermined range in a fixed period, the fuel can be fed at an appropriate fuel amount when the accelerator pedal is released or when the accelerator pedal is operated for sharp acceleration.
Since the gear changing operation is changed over between the gear changing pattern for lean-burn operation and the gear changing pattern for non-lean-burn operation, an optimum shift schedule can be designed according to the air-fuel ratio.
Since the gear changing operation and the lockup operation are executed according to the position of the accelerator pedal, the engine speed and the vehicle speed the line pressure of the automatic transmission is determined on the basis of the position of the accelerator pedal and the engine speed while the engine is in a lean-burn operation, and so optimum shift schedule, an optimum lockup schedule and an optimum line pressure can be determined even if the engine torque decreases during lean-burn operation.
Since the gear changing operation and the lockup operation are executed according to the position of the accelerator pedal and the vehicle speed, and the line pressure of the automatic transmission is determined on the basis of the position of the accelerator pedal in engine in which the air amount changes when the air-fuel ratio changes, optimum shift schedule, an optimum lockup schedule and an optimum line pressure can be determined even when the opening of the throttle valve does not correspond uniquely to the engine torque due to a change in a air amount resulting from the change of air-fuel ratio.
Since the air-fuel ratio command value is inhibited from specifying a lean air-fuel ratio when the temperature of the oil of the automatic transmission is lower than a predetermined temperature, or when it is found that a part of the automatic transmission is out of order, a lean air-fuel ratio is not determined when the temperature of the oil of the automatic transmission is comparatively low or when the automatic transmission malfunctions.